Parameters of cation transport and permeability in the crystalline lens of the rabbit eye are being correlated with Na-K-activated ATPase activity in different regions of the lens to give rise to a model which describes the relative role of the lens epithelium and lens fibers in the regulation of lens cation and water balance. Further studies concerning regional differences in Na-K-ATPase activity in relation to varying cation levels are proposed. In addition, the baseline characteristics of permeability and cation transport in mouse lens will be determined to enable us to undertake a study of these parameters during cataractogenesis that occurs in a given strain of mice.